Research On Certain Substance Trace Elements Of Cavity Enhanced Absorption Spectroscopy

With the development of industry and improvement of people’s living standard, human’s survival environment is suffering from various polluting gases. Therefore, we must take urgent and effective measures to monitor these gaseous pollutants. In the last two decades, Cavity Enhanced Absorption Spectroscopy and Cavity Ring-Down Absorption Spectroscopy are rapidly developed, widely applied and high sensitivity inner cavity trace gas spectral detection techniques. In fact, the working principle of Cavity Enhanced Absorption Spectroscopy(CEAS) is similar to that of Cavity Ring-Down Spectroscopy(CRDS) technique. Both of them have the advantages of long optical path, good detection sensitivity and spectral resolution etc. But compared to CRDS, The optical layout of CEAS is more simple and easy to manipulate. Furthermore, CEAS will eliminate the expensive photoelectric switch, and therefore reduces the experimental cost. So far, this technique has been applied to study interplanetary exploration, atmospheric chemistry, environmental monitoring, spectrum, the medical diagnostics and so on.This thesis consists of five chapters.The first chapter is an introduction. At the beginning, we briefly describe the knowledge of laser spectroscopic and then make a brief introduction of the techniques for trace gases detection, including traditional non spectroscopy methods and well developed laser spectroscopy techniques. Then the progress of cavity enhanced absorption spectroscopy at China and abroad is present. Finally we propose the main work of this study:based on the continuous wave cavity enhanced absorption spectroscopy technique, a cavity enhanced absorption spectroscopy apparatus has been built, and used to do experiments of N2O spectroscopy study and measurement. The second chapter introduces the basic knowledge of absorption spectroscopy (molecular absorption spectroscopy, atomic absorption spectroscopy, gas absorption spectroscopy), Bill-Lambert law and the relevant calculation and analysis of the absorption spectrum.The third chapter, the theory of the continuous wave cavity enhanced absorption spectroscopy (CW-CEAS) is described. The characteristics and optical cavity modes is discussed. Then working principles of the continuous wave cavity enhanced absorption spectroscopy techniques and the experimental apparatus are described in detail.In Chapter IV, the CEAS study and quantitative analysis of N2O gas are presented. First we make a brief introduction of the HITRAN database, the molecular structure of N2O and its spectra in HITRAN database. Based on the knowing near-infrared absorption spectrum of, N2O, the analytical line used for trace N2O detection and laser source are selected. By using the intense absorption line at6564.39cm-1, the Cavity Enhanced Absorption Spectroscopy of N2O gas mixtures centered at6561.39cm-1were obtained. The relationship between the absorption intensity and gaseous concentration was obtained by recording the absorption spectra of N2O gas mixtures at different concentration. The line-width of absorption spectroscopy vs the gas pressure and the detection limit had been discussed as well. Finally, the paper also discusses possibility for simultaneous analysis of nitrous oxide and carbon dioxide gas in a gas mixture. Indicating the cavity enhanced absorption spectroscopy apparatus estabilished here is a sensitive, easy manipulated and simplified technique, can be used for practical gas analysis.